As a fixing apparatus used in an image forming apparatus such as an electro-photographic copier or printer, there is a fixing apparatus for inserting a sheet of paper through a nip formed between a pair of rollers composed of a heat roller and a pressure roller or between similar belts and heating, pressurizing, and fixing a toner image. As such a heating type fixing apparatus, conventionally, there is an apparatus for heating a metallic conductive layer on the surface of a heat roller or a heating belt by the induction heating method. The induction heating method supplies predetermined power to an induction heating coil to generate a magnetic field, instantaneously heats the metallic conductive layer by an eddy current generated in the metallic conductive layer by the magnetic field, and heats the heat roller or heating belt.
In such a fixing apparatus of the induction heating method, for temperature control of the heat roller or fixing belt, as an apparatus for detecting the temperature without causing damage to the surface thereof, for example, in Japanese Patent Application Publication 2002-82549, an apparatus for arranging a temperature sensor on the inner peripheral surface of the fixing belt and controlling the temperature is disclosed.
However, this conventional temperature sensor is installed on the downstream side of the nip of the fixing belt and on the downstream side of an exciting coil. As a result, the real-time control of detecting the fallen temperature of the fixing belt at the time of fixing and before a sheet of paper reaches the nip next, heating and returning to a predetermined temperature cannot be executed. Further, the temperature sensor detects the temperature inside the fixing belt, so that there is a fear that an error may be caused between the detected result and the temperatures on both fixing sides.
Further, in Japanese Patent Application Publication 2003-35601, an apparatus for detecting the temperature on the surface side of an intermediate transfer belt generating heat by an exciting coil by a non-contact temperature detector is disclosed.
However, the non-contact temperature detector detects the temperature of the transfer belt reaching the exciting coil position and does not control the fallen temperature of the transfer belt at the time of fixing in real time before the next fixing time.
On the other hand, in recent years, a fixing apparatus of an induction heating method, a fixing apparatus for installing a thinned metallic conductive layer having a small heat capacity on the surface of a heat roller to realize faster heating of the metallic conductive layer and realizing more energy conservation has been developed. Such a heat roller having a thinned metallic conductive layer with a small heat capacity shows a greatly fallen temperature due to the fixing operation. Therefore, after passing the nip, before the same position of the heat roller next reaches the nip, unless the fallen temperature is recovered immediately by heating, the next fixing temperature at the same position of the heat roller is not sufficient. When the heating of replenishing the fallen temperature due to fixing is not in time, the difference in the surface temperature of the heat roller appears in a fixed image, and on the same image, temperature ripple marks different in gloss are caused, and the image quality is deteriorated.
Therefore, in the fixing apparatus having the installed thinned metallic conductive layer, development of a fixing apparatus of an image forming apparatus for immediately heating the heat roller after passing the nip, before the same position of the heat roller next reaches the nip, controlling the induction heating coil in real time so as to return the heat roller to the predetermined fixing temperature, having an excellent fixing property, and obtaining a high image quality is desired.